B2.044 Polyuria/Polydipsia Flashcards
what is normal daily urine output?
1-1.5 L/day
what is polyuria?
increased urine output
>3L/day
what is polydipsia?
excessive thirst and water ingestion
what 2 mechanisms account for water intake?
ingestion- 2000 ml/day
metabolism- 400 ml/day
what 4 mechanisms account for water loss?
skin- 400 ml/day
lungs- 400 ml/day
intestine- 100 ml/day
kidney- 1500 ml/day
what percentage of your body is water content?
60-65%
how is water content in the body distributed in tissues?
50% intracellular fluid
10% interstitial fluid
5% intravascular fluid
what is the blood pressure at the arterial and venous ends of the blood vessels?
arterial - 30 mmHg out of capillary
venous- 10 mmHg into the capillary
what is oncotic pressure?
pressure due to proteins in the body fluids
what is the net oncotic pressure difference?
20 mmHg into the capillary, constant along vessel
where is there a pressure difference favoring water efflux from vessel?
arterial end
where is there a pressure difference favoring water entrance to vessel?
venous end
what are the homeostatic functions of the kidney?
regulation of extracellular fluid volume
regulation of extracellular fluid electrolyte composition
regulation of extracellular fluid acid base balance
what are the excretory functions of the kidney?
metabolic waste products
foreign substances and toxins
describe the role of different nephron sections in water reabsorption/excretion
- blood is filtered in the glomerulus
- water is reabsorbed in the proximal tubule in a constitutive fashion
- 60-70% of water
- linked to Na+ - loop of henle not directly involved in reabsorption, but is fundamental in ADH dependent water balance
- distal tubule/collecting duct water reabsorption is regulated by ADH
what two components are maintained in regulation of water balance?
ECF volume
ECF osmolarity
when does the posterior pituitary release ADH?
increased ECF osmolarity
stimulation of thirst center and osmoreceptors
what is the effect of ADH on urine and plasma H2O?
decreases urine volume and increases urine osmolarity to save water in the plasma
once there is enough plasma H2O, there is negative feedback which inhibits ADH
a decrease in these metrics causes an increase in ADH
volume and pressure
an increase in this metric causes and increase in ADH
plasma osmolarity
what is the relative osmolarity and flow rate of urine during diuresis?
high flow
low osmolarity
what is the relative osmolarity and flow rate of urine during antidiuresis?
low flow
high osmolarity
what are the 2 factors required for water reabsorption in the kidney?
- permeability across the renal tubule membrane
2. a gradient for water movement across the tubule
how does ADH affect the distal tubule?
tubule impermeable
ADH increases permeability by creating channels for water to exit
what is the difference between the dlh and the alh?
dlh is permeable, alh is not
this allows for the creation of a more dilute fluid in the alh
describe the countercurrent mechanism
in the dlh, water exits and salt enters
in the alh, salt exits and water is maintained due to impermeability, creating a dilute solution
in the distal tubule and collecting tubule, there is now a gradient between the dilute filtrate and salty environment of the medullary interstitium so that water may flow out of the tubule if ADH is present and makes it permeable
what is the response of the kidneys and the resulting effect on urine of an event resulting in loss of water > solutes?
water reabsorption in kidney
decrease urine volume
increase urine conc. (hypertonic)
what is the response of the kidneys and the resulting effect on urine of an event resulting in excess water over solutes?
water excretion
increase urine volume
decrease urine conc. (hypotonic)
what is the response of the kidneys and the resulting effect on urine of an event resulting in excess of isotonic fluid?
water and solute excretion
increase urine volume
what are two mechanisms that lead to polyuria?
- low solute reabsorption
2. abnormal ADH levels
what is the effect of low solute reabsorption?
solute diuresis due to: -salt wasting -diuretics -high glucose in renal tubule
describe the mechanism for polyuria: solute diuresis
excess solutes in the filtrate (due to presence of osmolytes (DM), inhibition of salt reabsorption by drugs, or decreased ADH)
decreased water reabsorption in the proximal tubule
partial but insufficient compensation by distal tubule reabsorption
urine osmolarity isotonic or close to plasma
increased urine volume
what are the 4 components of diabetic nephropathy?
- thickening of the glomerular basal membrane due to accumulation of glycated proteins
- efferent glomerular arteriole atherosclerosis
- messangial and glomerular expansion
- disruption of podocytes
describe the mechanism of solute diuresis in DM
high plasma glucose
increased glucose filtration
glucose remains in tubule
increases diuresis (polyuria)
describe the mechanism for polyuria: water diuresis
excess water ingested decreased ECF osmolarity decreased ADH decreased nephron water permeability and reabsorption solute reabsorption continues increased urine dilution
what are the two components of diagnosing polyuria?
history and testing
what is the process for testing polyuria?
restrict water/raise plasma osmolarity
monitor urine osmolarity/volume and ADH levels in plasma
look at plasma and urine glucose levels
what should happen to urine volume/osmolarity after the addition of ADH?
should level off
what should happen to urine osmolarity as plasma osmolarity increases?
increase in a steep linear fashion
what should happen to urine volume as plasma osmolarity increases?
decrease in a steep linear fashion
how do presentations of nephrogenic diabetes insipidus and central diabetes insipidus differ on a polyuria test?
nephrogenic doesn’t react to ADH addition while central does
